Shelf



FIG. 1 is a front elevational view of a shelf showing my new design;

FIG. 2 is a rear elevational thereof;

FIG. 3 is a right side elevational view thereof, the left side elevational view thereof being a mirror image; and

FIG. 4 is a perspective view taken from the top at the right side front corner of the shelf, the shelf having the same appearance when viewed in perspective from the bottom. 

The ornamental design for a shelf, substantially as shown and described. 